OXYGEN-ISOTOPE SEDIMENTOLOGY OF GOLD-BEARING AND URANIUM-BEARING WITWATERSRAND AND HURONIAN SUPERGROUP QUARTZ-PEBBLE CONGLOMERATES

The relation between Au and U content and provenance has been evaluate d for a number of quartz-pebble conglomerates from the Archean Witwate rsrand and Early Proterozoic Huronian Supergroups, through analysis of the oxygen isotope composition of siliceous detritus within these roc ks. Oxygen isotope compositions of quartz, quartzite, and chert pebble s, as well as of individual sand-sized quartz grains in the matrix of the conglomerates, were measured using conventional and laser fluorina tion techniques. Significant heterogeneity in the delta(18)O values of quartz and quartzite pebbles and quartz sands within individual sampl es of these conglomerates, suggests that they have preserved delta(18) O values of their source rocks. Preservation of original delta(18)O va lues of individual sand grains in conglomerates whose matrix displays a typical metamorphic texture implies limited oxygen and silica mobili ty, and recrystallization by pressure solution in the presence of smal l amounts of intergranular fluids.Oxygen isotope analyses of quartz pe bbles along the Main ''reef'' (conglomerate) of the West Rand Consolid ated gold mine, Witwatersrand, indicate that quartz-pebble delta(18)O values reflect local variations in provenance. Yet, the overall delta( 18)O distributions for quartz pebbles and quartz sands from the Au- an d U-bearing Main reef and the directly overlying U-bearing Monarch ''r eef'' (conglomerate) in the same mine are identical, implying an overa ll similarity in their source rocks. Comparison of the quartz-pebble d elta(18)O distributions between these two reefs and other Witwatersran d conglomerates, including polymictic conglomerates from the Welkom ar ea of the Witwatersrand basin, moreover, suggests that the provenance was similar throughout deposition of much of the Central Rand Group. D ifferences in the ore content and Au/U ratio between conglomerates of the Central Rand Group are compatible with differences in their sedime ntology. In contrast to conglomerates from the Witwatersrand Supergrou p, delta(18)O distributions of quartz pebbles from different conglomer ates throughout the Huronian Supergroup indicate distinct differences in provenance. A erude positive correlation between Au content and pro portion of quartz pebbles with delta(18)O values greater than or equal to 11.5 per mil exists for these conglomerates. The U-bearing Matinen da Formation conglomerates in the Elliot Lake area have a very low pro portion of pebbles with delta(18)O values above 11.5 per mil. Matinend a Formation conglomerates sampled closer to present outcrops of Archea n greenstone terranes, as well as conglomerates of the Lorrain and Liv ingstone Creek Formations, have a higher proportion of quartz pebbles with delta(18)O values greater than or equal to 11.5 per mil and also higher Au contents. Differences between the delta(18)O distributions f or quartz sands and quartz pebbles help to identify sediment sources. Absence of quartz sands with delta(18)O greater than or equal to 11.5 per mil, but presence of quartz pebbles with such values in the same s ediment, may indicate a source which yields little or no sand-sized ma terial in fluvial settings. Comparison of the delta(18)O values of qua rtz pebbles and quartz sands in quartz-pebble conglomerates to delta(1 8)O values for quartz from likely source rocks suggests that compositi onally homogeneous granites and related pegmatites are the dominant so urce of uranium, whereas hydrothermal lode gold veins are the dominant source of gold.